This invention relates to a cut-off control system for variable displacement pumps and more particularly to a control system for preventing the generation of excessive pressure by reducing the displacement volume of such pumps.
In conventional control systems of the type specified, the arrangement is made such that when the delivery pressure of the pump has become more than a predetermined cut-off pressure set by a spring of a cut-off control valve, the spool thereof is urged against the biasing force of the spring so that the pump pressure is transmitted to the chamber of a variable displacement device so as to urge a piston mechanically connected to the variable displacement pump thereby reducing the displacement volume of the pump.
In this conventional system, there are following problems.
(1) Improvement in response to control system renders the system unstable. Such unstable condition is caused by overdisplacement of the variable displacement device.
(2) Because the displacement volume of the pump reaches its maximum when the pump is stopped, the prime mover for the pump cannot be easily started.
(3) Since the fluid pressure source for the control valve varies depending on the load applied, it becomes difficult to provide other types of displacement controls than the cut-off control.
(4) Heretofore, in case where a plurality of actuators are driven by a single variable displacement pump, the cut-off control value must be adjusted manually for each actuator in operation.